BY JAMES M. PETRIE. 837 



the results quoted above were so obtained. On the other hand, 

 the proteins of animal-extracts are usually separated by the 

 tannin-salt method. Alcohol is also known to be a valuable 

 precipitant, but has been very seldom used. 



Since the whole value of the results depends on knowing 

 ■exactly what is precipitated by the reagent in each of the above 

 methods, it is absolutely necessary to know the full details of 

 their work before we can compare the results of various experi- 

 menters. A further difficulty is met with in the fact that 

 -authorities are not agreed as to the completeness of the separa- 

 tion in each case, especially of proteoses and peptones. 



In choosing the most convenient and suitable method for the 

 work in hand, the following details were carefully considered. (1) 

 As to Stutzer's method(3) precipitating too little(4) — peptones are 

 not thrown down, or only very slightly; Stutzer, in his original 

 paper, states that they are carried down with the proteins and 

 ■cannot be washed out by water. He also states that albumoses 

 are completely precipitated. On the other hand, Schulze, who 

 has had great experience with this method, says that it is not 

 known for certain whether albumoses are precipitated or not, and 

 at least both proteoses and peptones are very incompletely 

 precipitated(5). Suzuki(6) came to the conclusion that only 

 certain groups of proteoses were precipitated, and in his experi- 

 ments with the Lima-bean found the method unsuitable. 

 Mellanby's(7) experiments with serum-proteins show that pre- 

 cipitation with copper salts was never complete; he could not 

 •obtain more than 85 % of the protein. Next as to Stutzer's 

 method precipitating too much, various authors have stated that 

 the purin bases are partially precipitated, and we find that 

 Schulze precipitates amides by warming with copper hydroxide. 

 These are two highly important groups of substances in plant- 

 extracts, and it is essential that we know exactly how they act 

 towards the reagents. 



(2) Tannic acid and salt solution was supposed to precipitate 

 proteins and proteoses completely, while peptones are soluble in 



